1. Field of the Invention
The present invention relates to an interferometer such as an interference type encoder.
2. Description of the Related Art
A conventional diffraction interference type rotary encoder needs to be adjusted to a so-called “one color state” by checking the state of interference fringes in an image when mounting the encoder on a mechanical member including a plurality of adjustment mechanisms for the spacing between the head unit and the disk unit, a positional shift in the radial direction, the tilt of the head, and the like. In this specification, a “one color state” is defined as a state in which the light/dark phases are uniform on the entire surface of the light-receiving unit for interfering light. For example, the interference between ideal plane waves corresponds to a case in which the angle difference between the principal rays of two light beams is 0°. The interference between spherical waves corresponds to a case in which the virtual point light sources of two light beams coincide with each other in a space. The interference between wavefronts with some distortions corresponds to a state in which the wavefronts of two light beams completely overlay each other. In addition, a “one color degree” is defined as the number of interference fringes (including numbers after the decimal point) on the light-receiving surface, which is defined as 0 in a perfect one color state. When the size of a light beam is small relative to the light-receiving unit, a one color degree is applied to the size of the light beam. In addition, one color degrees are respectively defined relative to two coordinate axes V and H on the light-receiving surface. For example, these one color degrees will be referred to as a “V-axis direction one color degree” and an “H-axis direction one color degree”, respectively.
When mounting a module type encoder with a separate head and disk in an apparatus, the user cannot acquire any interference fringe image as information inside the encoder. For this reason, in general, the mounting posture of an encoder is adjusted, based on only the amplitude of a periodic signal output from the encoder head, so as to maximize the amplitude. This applies to general geometric optics module type encoders as well as diffraction interference type encoders. For example, Japanese Patent Laid-Open No. 5-133732 discloses a technique of allowing a user to find a better adjusted state by presenting information indicating the quality of a mounted state based on the amplitude of a periodic signal.
According to the prior art, however, techniques to adjust interferometers have not been standardized and have required the intuitions and experiences of users who perform mounting operation, in order to adjust the interferometers to optimal states. That is, for example, since it is not clear which part is to be adjusted to what extent, the user keeps searching for a position where a better signal state is obtained, by determining an increase/decrease in signal upon actual adjusting operation. Interferometers and high-precision diffraction interference type encoders, in particular, need to be adjusted to an ideal one color state. However, such adjustment has depended on the skill of the user who performs mounting operation.